[...] I have now looked a little into your pianoforte arrangement [of Tristan]. What I often regret is such frequent fullness at the extreme parts; perhaps you are too scrupulous about the inner detail. The bass is often much too slight, and reminds me of the old timid methods of arrangement. I think it would frequently be an improvement if the arrangement were in two parts only, with upper and lower parts in octaves. As it is, your caution tends to make it too weak and, in particular, I am continually irritated by the "consumptive" basses. I think that a pianoforte arrangement of this sort needs far more summary treatment. Nothing very clear emerges for the musician from the middle parts as arranged for the pianoforte. Still, all this may be easier to say than to do. [...]
Source: Letters of Richard Wagner, Vol. 2, selected and ed. Wilhelm Altmann, trans. M. M. Bozman (J. M. Dent & Sons, 1927), No. 443